Methacrylic resins have excellent transparency, light resistance, surface hardness and so on. Various optical members, such as light guide plates, lenses and the like, can be obtained by forming methacrylic resin compositions comprising the methacrylic resins.
In recent years, a demand for lightweight and wide liquid crystal displays is high, and optical members are also required to be reduced in thickness and to be increased in area according to the demand.
The clearance between optical members is reduced with the reduction in thicknesses of liquid crystal displays, and rubbing between optical members readily occurs in, for example, the production process. Consequently, it is necessary to prevent scratching due to rubbing.
In addition, optical characteristics such as refractive index and retardation are demanded to have higher degrees of precision with an enhancement in the image quality of displays. However, thin and wide molded articles tend to reduce the degrees of precision of the optical characteristics with the dimensional changes due to, for example, heat. Accordingly, the methacrylic resin composition as a raw material of optical members is highly demanded to have high heat resistance, in addition to high transparency and high moldability.
As a methacrylic resin composition having high heat resistance, for example, Patent Document 1 discloses a methacrylic resin composition comprising a methacrylic resin having a high syndiotactic ratio. Unfortunately, since the methacrylic resin is produced by emulsion polymerization, it is difficult to achieve high productivity, and high transparency cannot be obtained.
In typical methacrylic resins, acrylates, such as methyl acrylate, are copolymerized in an amount about several percent by mass, in order to prevent poor appearance of molded articles due to depolymerization during molding. However, the heat resistance tends to decrease with an increase in the ratio of the copolymerized acrylate. In order to enhance the heat resistance, it is desirable to prepare a methacrylic resin as a substantial homopolymer by decreasing the ratio of the copolymerized acrylate as much as possible. Example 3 of Patent Document 2 discloses an example of the method of producing a homopolymer of methyl methacrylate substantially not comprising any structural unit derived from acrylates.
However, in homopolymers of methyl methacrylate, depolymerization of the polymer itself, called a zipper reaction, generally occurs in a temperature range for molding. Consequently, a monomer of methyl methacrylate remains in the resulting molded article, which causes a problem such as poor appearance due to occurrence of silver. For the methacrylic resin obtained by the method described in Example 3 of Patent Document 2, there is no description about what influence occurs in depolymerization during molding by not copolymerizing any acrylate. In addition, such a methacrylic resin has insufficient fluidity and is therefore difficult to be formed into a thin and wide product having good appearance.
Patent Document 3 discloses a methacrylic polymer having excellent thermal stability obtained by polymerizing a monomer mixture composed of 90 to 100% by mass of methyl methacrylate and 0 to 10% by mass of a C1-8 alkyl acrylate copolymerizable with methyl methacrylate, wherein the proportion of the terminal double bonds of the polymer is adjusted to 5% or less, and the amount of the combined sulfur in the polymer is adjusted within a specific range. Although the methacrylic polymer has improved heat resistance, Patent Document 3 does not disclose any means for enhancing the fluidity and preventing scratching due to rubbing.